tools: kwboot: Fix parsing UART image without data checksum

The 32-bit data checksum in UART image is not checked by the BootROM and
also Marvell tools do not generate it.

So if data checksum stored in UART image does not match calculated checksum
from the image then treat those checksum bytes as part of the executable
image code (and not as the checksum) and for compatibility with the rest of
the code manually insert data checksum into the in-memory image after the
executable code, without overwriting it.

This should allow to boot UART images generated by Marvell tools.

Signed-off-by: Pali Rohár <pali@kernel.org>